1. Field of Invention
This invention relates to a contactor and trip circuit therefor. The contactor is operative for switching one or more high current loads between one or more electrical sources, for example an AC generator and DC batteries to ensure a constant electrical supply. A trip circuit is provided having an overload trip capability. The contactor, which may be in the form of a solid state contactor, may be used for providing interrupt free direct electrical power of, for example 150 Amps, to individual DC busses on an aircraft.
2. Description of the Prior Art
Conventional contactors may be provided with a trip capability for terminating the supply of current to a load when it exceeds a nominal operating level by a predetermined amount.
It is an aim of the present invention to provide a contactor which is capable of providing an indication of near trip events, that is, instances where the load exceeds the nominal operating level but not by enough or for long enough to trip the supply. It is therefore an aim of the present invention to provide a contactor having a xe2x80x98pre-tripxe2x80x99 history capability.
According to the present invention there is provided a contactor comprising a switching circuit for switching a load between electrical supplies, a current detector for detecting the current flowing through the load, a trip circuit for tripping the current after an interval of time which depends upon how much the current flowing through the load exceeds a predetermined amount, and a pre-trip circuit for providing an indication during said time interval that the current exceeds said predetermined amount for providing xe2x80x98pre-tripxe2x80x99 or xe2x80x98near-tripxe2x80x99 information.
The predetermined amount may be 10% above the maximum current which the contactor can carry without overload.
The pre-trip or near-trip information may be supplied to a monitoring computer for monitoring pre-trip activity. Embodiments of the invention are advantageous in that they permit monitoring of events which give rise to an overload current which are insufficient in duration or quantum to cause the contactor to trip but which nevertheless are indicative of the state of an overload condition.
In a preferred embodiment, the trip circuit includes a function generator for converting the sensed current flow to a signal current proportional to the required interval of time. The function generator circuit preferably has a response time characteristic which varies depending on how much the current exceeds the predetermined amount. The response time characteristic is preferably such that the time from when the current breaches the predetermined amount to occurrence of the eventual trip decreases as excess of the current over the predetermined amount increases. The time characteristic preferably resembles the I2T relationship of a bimetallic strip in which the heating characteristic of the strip varies with the square of the current I flowing through it (temperature is proportional to the square of the current multiplied by the resistance), where T is the trip time.
The trip circuit may be configured to generate a digital-type output representing respective ones of a plurality of trip time intervals each corresponding to a sub-range of I2T overloads. The function generator circuit may be provided with a pulse generator for generating pulses at a frequency which depends on the range of current overload. A counter is provided for generating a trip signal when a set number of pulses have been counted, the time interval being a function of the pulse frequency and the set number. The set number and frequency can be varied depending upon the required trip time interval for a given current overload. Conceivably, alternative parameters may be used to govern the time intervals, such as pulse amplitude and/or width in addition to or instead of frequency.
The function generator may include a current indicator for indicating the presence of current flowing through the load at levels below the nominal value. For example, the indicating means may indicate current at 10% of a 150 Amp nominal rating. An absolute overload current detector may additionally be provided for detecting substantial over currents arising from, for example, short-circuits and the like. The absolute overload current detector can be operative to cause the contactor to trip instantly. The detector may be configured to respond to overload currents in the order of 1000% of the nominal rating.
The current detector preferably includes two Hall effect transducers which sense the current in a power stage to which the contactor is coupled. One of the Hall effect transducers has a relatively high sensitivity for sensing low level currents and the other a lower sensitivity for high currents. Alternatively, a single resistive sensor may be used. Hall effect transducers are chosen with ranges to improve measurement accuracy around the trip threshold. Additional calibration may be necessary for alternative sensors.
The contactor embodying the present invention is provided with a trip recovery delay operative for holding the tripped state for a predetermined period of time to allow for cooling. After the trip recovery delay, the tripped contactor may be manually overridden or operated under control of an external computer.
The predetermined period of time may vary in dependence upon the extent of the overload which gave rise to the trip.
According to the present invention, there is also provided a trip circuit for a contactor, the trip circuit comprising means for tripping a supply of current after an interval of time which depends upon how much the current flowing through a load exceeds a predetermined amount, and a pre-trip circuit for providing an indication during said time interval that the current exceeds said predetermined amount for providing xe2x80x98pre-tripxe2x80x99 or xe2x80x98near-tripxe2x80x99 information.
Embodiments of the invention have the advantage that they provide for a quantitative monitoring of pre-trip events for diagnostic purposes.